1. Field of the Invention
This invention relates to concrete accelerating additives and, in particular, to a liquid accelerating additive ideally suited for shotcreting applications.
2. Brief Statement of the Prior Art
Precise acceleration of the setting of concrete is critical in shotcreting applications wherein an anhydrous concrete mixture containing relatively large aggregate, e.g. 1/4 inch or larger, is prepared outside a mine or tunnel and conveyed to a pneumatic dispenser inside the mine or tunnel. Water is then injected at the nozzle of the dispenser and the liquid concrete is sprayed at the freshly exposed walls of the mine or tunnel. An accelerating additive is incorporated in the concrete to control its setting time. If the concrete sets too fast, the aggregate in the concrete will have the tendency to fall out of the concrete when the concrete is sprayed onto the freshly exposed walls. This is known in the art as "rebound". If the concrete sets too slowly, the entire concrete mass will slump and/or fall from the walls. Thus, in shotcreting, it is necessary to have an accelerating admixture which provides the precise acceleration to the setting of the concrete.
Various accelerating additives have been developed for the setting of wet concrete or mortars in applications other than shotcreting and typical of these are calcium chloride and sodium carbonate. These materials are not satisfactory for use in shotcreting applications because they have an excessively high heat of hydration, resulting in premature drying of the concrete and cracking of the concrete surfaces and/or clogging of the spray nozzles used to apply the coating. Sodium aluminate of a regulated alkalinity has been suggested as a useful accelerating additive in U.S. Pat. No. 3,140,956, and the use of a metal aluminate in combination with an organic acid is suggested as an accelerating additive in U.S. Pat. No. 3,656,985.
While the sodium aluminate is superior in acceleration of concrete setting over other accelerating materials such as calcium chloride and sodium carbonate, the material is still not ideally suited for shotcreting applications. It is desirable to employ a liquid accelerating additive that can be continuously metered into the wet concrete mixture or the water ingredient thereof to provide a precise control over the setting of the wet concrete. Sodium aluminate also lacks an adequate shelf stability to be used, stored and handled as a liquid. Additionally, sodium aluminate does not provide adequate acceleration of the setting of wet concrete for optimum shotcreting applications.